Cryptographic operations are used for a variety of processes such as data encryption and authentication. In a typical symmetric cryptographic process, a secret key is known to two or more participants, who use it to secure their communications. In systems using asymmetric (or public key) cryptography, one party typically performs operations using a secret key (e.g., the so-called private key), while the other performs complementary operations using only non-secret parameters (e.g., the so-called public key). In both symmetric and asymmetric cryptosystems, secret parameters must be kept confidential, since an attacker who compromises a key can decrypt communications, forge signatures, perform unauthorized transactions, impersonate users, or cause other problems.
Methods for managing keys securely using physically secure, well-shielded rooms are known in the background art and are widely used today. However, previously-known methods for protecting keys in low-cost cryptographic devices are often inadequate for many applications, such as those requiring a high degree of tamper resistance. Attacks such as reverse-engineering of ROM using microscopes, timing attack cryptanalysis (see, for example, P. Kocher, "Timing Attacks on Implementations of Diffie-Hellman, RSA, DSS, and Other Systems," Advances in Cryptology--CRYPTO '96, Springer-Verlag, pages 104-113), and error analysis (see, for example, E. Biham and A. Shamir, "Differential Fault Analysis of Secret Key Cryptosystems," Advances in Cryptology--CRYPTO '97, Springer-Verlag, 1997, pages 513-525) have been described for analyzing cryptosystems.
Ciphers and algorithms believed to be cryptographically secure are known in the background art. For example, protocols using triple DES (a cipher constructed using three applications of the Data Encryption Standard using different keys) can resist all feasible cryptanalytic attacks, provided that attackers only have access to the standard inputs to and outputs from the protocol. However, even a product using an extremely strong cipher such as triple DES can be insecure if the keys are not managed securely.
This document assumes a detailed understanding of the Data Encryption Standard (DES), which is defined in Federal Information Processing Standards Publication 46 and need not be described in detail here. Information on DES and other cryptographic algorithms can also be found in Applied Cryptography by Bruce Schneier (Wiley and Sons, Inc., 1996), in the Handbook of Applied Cryptography by Menezes et al. (CRC Press, Inc., 1997), or in other standard references as will be appreciated by those skilled in the art.